This invention relates to a method and apparatus for performance management in a multiplexed transmission system, particularly but not exclusively an optical transmission system.
The demand for high speed and high capacity data transmissions has been rising. In long haul transport, as well as in metro ring applications, the use of dense wavelength-division-multiplexing (DWDM) or wavelength-division-multiplexing (WDM)allows for increases in the transmission bandwidth by two, three or more times.
DWDM, or equivalently, for the purposes of this specification, WDM, systems permit a number of signals to be carried along a single optical fiber by modulating each signal about a separate optical carrier wavelength. Typically, optical routing and signal regeneration are performed by passive and active optical elements.
DWDM optical fiber telecommunication systems can have extremely high overall data capacity since each channel is capable of carrying a high data rate signal. These high capacity signals can be carried cost-effectively over many hundreds of kilometers if Erbium Doped Fiber Amplifiers (EDFA) are used to boost the power of the optical signal periodically and overcome the loss incurred in the optical fiber and the passive optical elements. There is a growing requirement to increase the capacity of the existing communication systems.
While optical amplifiers are designed to produce a linear gain profile, as a practical matter, the wavelength dependent profile of EDFAs and other optical elements in the network is non-uniform, so that this objective cannot always be reached across the entire wavelength range over which signals will be transmitted. A significant challenge in carrying such multi-channel signals over many spans of fiber separated by boosting EDFAS has to do with the fact that the wavelength spectrum of the gain of the EDFAs is not flat. In fact, because of the physical properties of the Erbium ions that provide the gain, the shape of the gain spectrum changes from strong gain (about 23.5 dB at 1530 nm) to weak gain (about 21.5 dB at 1560 nm). The fiber span also shows non-uniform loss across the wavelength spectrum. Generally, the higher the wavelength, the higher the loss.
Moreover, even with a gain flattening filter, the gain profile of an optical amplifier across the wavelength range still shows ripple and gain tilt. The ripple (the slight variation in the gain) and tilt (the slope of the gain profile) are functions of input power and are intrinsic properties of the amplifier material.
Further, due to aging amplifiers and environmental factors, optical signal quality could degrade, resulting in a degradation of the system performance over time.
In a long multi-span cascade of fiber spans and EDFA line-amplifiers, the nominal gain of the EDFA is set equal to the span loss, so that a nominal channel does not rise or fall in power as it propagates downstream. This non-ideal gain (due to EDFAs) and loss (due to fiber and component lose) spectrum means that in a long multi-span cascade of fiber spans and EDFA line-amplifiers, some channels will have more gain or higher loss than the average and will grow in relative power as the multi-channel signal propagates down the link. However, some channels have less gain or lower loss than the average, and so the power of that channel will decrease as the multi-channel signal propagates down the link.
The non-linear nature of the overall gain or loss profile has a profound impact on the bit error rate (BER) of the optical link.
The amount of gain provided by an EDFA is controlled by the amount of pump laser power that is applied to the Erbium doped fiber, and typically covers a range of 15 dB to 35 dB. The amount of output power capability of the EDFA is also influenced by the amount of pump laser power. For any given amount of pump power, there is a certain limit to the total power over all of the channels, with 15 dBm as an example of a typical value. This is a natural physical limit at which the pump photon flux is just sufficient to replenish the depletion of the Erbium population inversion by the high signal output power. As well as this natural physical limit on the total power capability, there can also be an additional lower limit applied by design. For a given number of channels, it might be useful to limit the total power out of the EDFA and launched into the optical fiber in order to avoid certain nonlinearities in the fiber. This total power control (TPC) mode typically is implemented by tapping off a very small but controlled fraction of the light at the output of the EDFA and monitoring that with a photodetector.
Since all of the wavelength channels can carry revenue generating traffic, it is of interest to ensure that all of the channels meet a certain standard of performance. In a digital system, BER is typically used as a figure of merit, and 10xe2x88x9212 is a common objective for BER. One of the main influences which will degrade the BER of multi-span EDFA links is the noise known as the Amplified Spontaneous Emission (ASE) which is generated inside the EDFAS. The amount of total noise (ASE, signal-to-spontaneous beat noise, spontaneous-to-spontaneous noise, etc.) relative to the signal power is typically quantified by the Optical Signal to Noise Ratio (OSNR), defined as:
OSNR=Signal Power/(noise density*BWOSNR)xe2x80x83xe2x80x83(1)
where BWOSNR is the spectral band over which the OSNR is defined (for example 0.1 nanometers).
To optimize the OSNR of any given channel in a multi-span link, the input powers to each EDFA should be kept as high as possible at all of the amplifiers. This influences the design of multi-channel links where some channels will be increasing in power going down span, and some channels will be decreasing in power. The simplest case to consider is one in which all of the channels are initially launched at the same power. In the case of a channel which has more than average EDFA gain, it increases in power after that initial launch point, up until the receiver. With such high powers going into the EDFAs, that channel will have a good OSNR and will then have a good BER, provided that fiber nonlinearities are not provoked. However, a channel which has less than average gain will drop in power at every span as it propagates down-link. This channel will have a poor OSNR and thereby will have a high BER, which may not meet an objective like 10xe2x88x9212.
At first, it might be thought that the simplest way to ensure that the weak channels do not severely hamper the system would simply be to turn all transmitters up to their highest launched power achievable. However, constraints (either natural or by design) on the total power available from the EDFA rule out this simple approach. Given that the total EDFA power is limited, the solution in the past has traditionally been to turn up all transmitters only by the appropriate amount such that the end performance (either OSNR or BER) is balanced between all channels. If any transmitters were launching more than the power necessary to achieve this balanced performance condition, then they would necessarily be taking more power than they need from at least one of the EDFAs. Because of the constraint on total EDFA power, this removal of power would then reduce the power available to the weaker channels. This means that the performance of the weaker channels would suffer if the strong channels were allowed to get better end performance than the average. In conclusion, when operating under total power constraints, adjusting the channel launched power of the transmitters to achieve equalization of the end performance of all of the channels is the optimum solution.
Therefore, it is important to have a method to adjust the launching power of the channels in order to equalize the BER performance of all the channels. Since aging and optical degradation happens over time, it is important to develop the equalization method so it can be used during the operation of the network and not simply during system set up.
One solution to this problem would be to physically measure the BER value generated at a receiver and use this information to adjust the launching power of all the channels to provide equal output BER. Such an approach, has not been heretofore practical, however, because of the long monitoring times that would be required in order to calculate the receiver BER, for BER values representative of a channel with any practical value.
Another solution to this problem would be to determine the OSNR values corresponding to each DWDM channel of the DWDM signal being received at a DWDM receiver and to subsequently attenuate the input power of the DWDM channels with high OSNR at the transmitter prior to multiplexing the channels.
In U.S. Pat. No. 5,225,922, (Chraplyvy et al.) issued Jul. 6, 1993 to ATandT Bell Laboratories entitled xe2x80x9cOptical Transmission System Equalizerxe2x80x9d, the OSNRs at the output of an amplified WDM system are measured directly and the input powers are iteratively adjusted to achieve equal OSNRs.
However OSNR values alone do not accurately characterize the system performance. Rather, OSNR is only one of several parameters that affect the performance of an optical transmission system, which by definition, is fully expressed by the BER.
In European published Patent Application No. EP 0926 854 A2 (Barnard et al,) laid-open for publication on Jun. 30, 1999 and entitled xe2x80x9cMethods for Equalizing WDM Systemsxe2x80x9d, there is disclosed a method of equalizing the channels of a WDM link by identifying for each optical channel in the link an error threshold level for the BER of the optical channel and the attenuation of the channel""s power along the link and then adjusting the input powers of the weaker channels in accordance with the measured attenuations of all of the channels to obtain substantially equal BER for all of the channels.
With both of these approaches, however, the system is unable to operate while data is being sent on the network since both of these algorithms must at times make sure channels are accessible (i.e. taken off service) in order to determine constants used within the attenuation calculations.
Accordingly, it is desirable to provide an improved method and apparatus for optical channel performance management.
It is further desirable to provide a method and apparatus for optical channel performance management that can be implemented while the system is in operation.
The present invention accomplishes these aims by providing a forward error correcting (FEC) element within a WDM receiver, which provides as an output, a signal having a BER which provides an improvement over the BER of the signal provided at its input in a known relationship. As a result, the BER at the input is sufficiently large that it can be measured and used for performance management purposes in real-time while maintaining the low BER performance required for practical operation of a WDM system.
According to a broad aspect of an embodiment of the present invention, there is disclosed a method of equalizing the performance of a plurality of multiplexed transmission channels comprising: encoding signals for transmission in the channels using a forward error-correcting code; receiving the encoded signals; determining the BERu (bit error rate prior to forward error correction) of each of the received signals; decoding the received signals using the forward error-correcting code to retrieve output data signals; and adjusting the transmission powers of the channels according to the determined BERu for each channel thereby to equalize the BERu across the channels.
According to a second broad aspect of an embodiment of the present invention, there is disclosed a communications system comprising at least two transmitters for transmitting respective signals to respective receivers in a multiplexed signal across a communications channel, the transmitters and receivers respectively coding and decoding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in the receiver in a known relation to the BER before forward error-correction, a system manager comprising: a status module for receiving, from the receivers in the communications system, the BER of their respective signals before forward error-correcting decoding; a calculation module for determining the relative launch power of each transmitter which will provide optimal BER performance of the multiplexed signal along the communications channel; and a command module for issuing commands to the transmitters in the communications system to adjust their launch powers in accordance with the relative launch powers determined by the calculation module.
According to a third broad aspect of an embodiment of the present invention, there is disclosed a communications system comprising a system manager and at least two transmitters for transmitting respective signals in a multiplexed signal across a communications channel, the transmitters coding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in a known relation to the BER before forward error-correction, a receiver associated with each transmitter for receiving its respective signal comprising: a decoder for decoding the received signal using the forward error-correcting code; a BER calculator for determining the BER of its respective signal before forward error-correcting decoding; and a communications module for providing the BER to the system manager; whereby the system manager may determine, from the BER values provided to it, the relative launching power of each transmitter required to optimize the BER performance of the multiplexed signal along the communications channel; and each transmitter in the communications system may adjust its launching power in response to commands from the system manager.
According to a fourth broad aspect of an embodiment of the present invention, there is disclosed a communications system comprising a system manager and at least two receivers for receiving respective signals from a multiplexed signal send across a communications channel, the receivers decoding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in a known relation to the BER before forward error-correction, a transmitter associated with each receiver for transmitting its respective signal comprising: a encoder for encoding its respective signal using the forward error-correcting code; and a launch power adjustment module for adjusting the launch power used to transmit the signal in response to commands from the system manager, whereby each receiver may determine the BER of its respective signal before forward error-correcting decoding and provide the BER value so determined to the system manager; and whereby the system manager may determine, from the BER values provided to it by each receiver, the relative launching power of each transmitter required to optimize the BER performance of the multiplexed signal along the communications channel and issue commands to each transmitter in accordance therewith.
According to a fifth broad aspect of an embodiment of the present invention, there is disclosed a computer-readable medium storing computer-executable program instructions which, when executed by a processor in a system manager in a communications system comprising at least two transmitters for transmitting respective signals to respective receivers in a multiplexed signal across a communications channel, the transmitters and receivers respectively coding and decoding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in the receiver in a known relation to the BER before forward error-correction, cause the system manager to: receive, from the receivers in the communications system, the BER of their respective signals before forward error-correcting decoding; determine the relative launch power of each transmitter which will provide optimal BER performance of the multiplexed signal along the communications channel; and issue commands to the transmitters in the communications system to adjust their launch powers in accordance with the relative launch powers determined by the calculation module.
According to a sixth broad aspect of an embodiment of the present invention, there is disclosed a computer-readable medium storing computer-executable program instructions which, when executed by a processor in a receiver in a communications system comprising a system manager and at least two transmitters for transmitting respective signals in a multiplexed signal across a communications channel, the transmitters coding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in a known relation to the BER before forward error-correction, cause the receiver, being associated with one of the transmitters for receiving its respective signal, to: decode the received signal using the forward error-correcting code; determine the BER of its respective signal before forward error-correcting decoding; and provide the BER to the system manager; whereby the system manager may determine, from the BER values provided to it, the relative launching power of each transmitter required to optimize the BER performance of the multiplexed signal along the communications channel; and each transmitter in the communications system may adjust its launching power in response to commands from the system manager.
According to a seventh broad aspect of an embodiment of the present invention, there is disclosed a computer-readable medium storing computer-executable program instructions which, when executed by a processor in a transmitter in a communications system comprising a system manager and at least two receivers for receiving respective signals from a multiplexed signal send across a communications channel, the receivers decoding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in a known relation to the BER before forward error-correction, cause the transmitter, being associated with each receiver for transmitting its respective signal, to: encode its respective signal using the forward error-correcting code; and adjust the launch power used to transmit the signal in response to commands from the system manager, whereby each receiver may determine the BER of its respective signal before forward error-correcting decoding and provide the BER value so determined to the system manager; and whereby the system manager may determine, from the BER values provided to it by each receiver, the relative launching power of each transmitter required to optimize the BER performance of the multiplexed signal along the communications channel and issue commands to each transmitter in accordance therewith.
According to a eighth broad aspect of an embodiment of the present invention, there is disclosed a method of adjusting the performance of a channel in a multiplexed transmission environment comprising: encoding signals for transmission in the channel using a forward error-correcting code; receiving the encoded signal; determining the BERu (bit error rate prior to forward error correction) of the received signal; decoding the received signal using the forward error-correcting code to retrieve the output data signal; and adjusting the transmission power of the channel according to the determined BERu for the channel thereby to establish a BERu proportionate to the relative importance of the signal to other signals in the multiplexed transmission environment.